In New Zealand reports of whales stranding on the beaches make headlines, such as the recent stranding of pilot whales at Spirits Bay.[1] They’re mammals like us and New Zealanders have grown up with news stories of whales and their inquisitive cousins, dolphins. It’s pitiful to see these creatures sprawled helpless on the sand.

Frequent readers will know that I have an interest in deafness-related issues, so while not knowing a lot about these mammals that swim in our waters I was intrigued by a recent paper in Public LIbrary of Science exploring the idea that dolphins that beach might be suffering hearing losses.

Like most New Zealanders I’ve seen whales and dolphins in open waters at first hand.

My encounters with dolphins and, less often, whales have been from boats. Dolphins love to play in the pressured water of the bow wave of a moving boat. It’s great fun to lie at the bow looking at them swim and leap a few metres away from you.

As a child, I saw a killer whale closer than I’d like in a small yacht,[2] which was a little unsettling with their reputation as a hunter. As an adult I recall standing at the stern of a cruising yacht, probably about 28 foot in length (~8.5m), off the Kaikoura coast and seeing the fluke of a large whale off the stern. Another crew member at the same time reported a whale sighting ahead of the bow – the same whale as it turned out. The size of the larger species is impressive when seen up-close.

Dolphins and whales use echolocation to sense objects for navigation and foraging for food. They reasoned if they were deafened it might explain some of the beachings and embarked on testing the hearing of recently beached dolphins.

David Mann and his colleagues from Florida and the Netherlands investigated if beached dolphins in Florida were deaf.

I suppose you’re like me wondering how on earth do you test the hearing of a dolphin?

Hearing tests have advanced considerably since I was a kid. Then there were several set-ups, all somewhat cumbersome. The simplest was to sit facing the corner while a list of words was read to you. (Words differing only by the leading consonant or a vowel can inform the tester about specific hearing losses.) In another I was sat in a booth, with the tester opposite me, sometimes behind the glass of an adjacent room. The tester would feed sounds to heavy headphones and I’d respond by pressing a button or turning a switch. (This test is subject to kids who want to please cheat by trying too hard, watching the tester’s actions closely when they fiddle the knobs to generate the sounds!)

Both these tests require that the tested person respond.

A more modern approach is to use auditory evoked potentials, which measure electrical activity in response to sounds.

You will have heard of ECGs, the heart monitors shown in all those TV hospital dramas that detect small changes in electrical charge on the skin as the heart beats. Electroencephalograms – EEGs – are conceptually similar, but measure neuronal (nerve cell) activity in brains.

Researchers can measure response to a stimulus by evoking the stimulus, perhaps a sound, and measuring the change in electrical potential, hence auditory evoked potentials.

A key to this is that it requires no response from the patient. For us humans, babies have their hearing tested this way. It’s also quicker than testing using responses.

Whereas in humans a thin electrode wire is carefully threaded into the ear, for the dolphins sounds were delivered by a transducer (’speaker’) within a suction cup placed on the lower jaw. While dolphins have small ear opening behind their eyes, dolphins are though to mainly sense noise vibrations through the bone of their lower jaws.[3]

The response to the sounds were measured using electrodes embedded within a suction cup placed approximately 5-10cm behind the blowhole, with a reference electrodes recording ~40cm behind the blowhole.[4]

The researchers’ results differed with different species of dolphins tested, but they observed severe (~70-90dB loss) to profound (>90dB loss) in four of seven (57%) of bottlenose dolphins and 5 of 14 roughtooth dolphins (36%). The sample sizes are small,[5] but they stood in contrast to essentially no recorded hearing losses in population health studies of more than 60 bottlenose dolphins from the same region.

Dolphins with profound hearing loss were evenly split across the sexes.

One confounding factor they note is antibiotic-induced hearing loss. A number of antibiotics are known to be able to induce deafness in mammals through cochlear hair cell damage, and apparently antibiotics (and anti-fungals) are given to stranded animals. They report this was unlikely to confound their data as some of their tests were done immediately after stranding, but they raise it as a concern that might prohibit successful release.[6]

I cannot comment on how successful hearing tests on dolphins might be – it strikes me that these must be fairly tricky to get right in the field, and I would welcome comments from marine biologists on this paper. Nevertheless it intrigues me that deafness might play a part in the beaching of these mammals.

Footnotes

1] The Science Media Centre, who host our blogs, have a podcast of local scientists presenting their thoughts on whale stranding. I haven’t listened to this podcast. My hearing isn’t very good – I have a hearing loss, hence my interest in deafness-related news – and at a touch over one hour long I haven’t the time either! But don’t let that stop you. I’d be interested to hear if any of the speakers mention hearing loss as a possible reason for stranding.

[2] My recollection was that it was a Sunburst, which at 3.5m long are quite a bit smaller than an adult killer whale (4.5-10m).

[3] There are suggestion that the regular array of teeth in the lower jaws of dolphins act like an array of sonar antenna, aiding location of the signal source, but this is disputed. For an interesting account of this and sonar in other species, see Biological sonar systems by Mo Costandi. Mo also writes at the Guardian blogs.

[4] In my personal experience the responses were measured with the recording electrodes placed over mastoid bone behind the ear. One source of information is a manufactures tutorial guide (large PDF) for using the procedure.

[5] To someone used to molecular biology data.

[6] One confounder they don’t mention that occurs to me, is if pollution can induce hearing loss in marine mammals – ?

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I was going to share the joke about how we know spiders have ears on their legs. It seems easy to adapt to this problem.

Orcinus Orca 918 days ago

As the majority of Cetacean hearing range is above and beyond that of Humans, I am curious as to what frequency range is being tested in the Cetaceans by this. How is a baseline reference determined for a Cetacean with ‘normal’ hearing?

There are several problems with this study. First only 11 out of 36 tested dolphins had some issues with hearing (Table S1). Out of these 11 authors did not specifically explained which ones were treated with antibiotics and which were not. The study would have more meaning if authors stated which ones were treated prior to hearing tests. Each organism reacts differently and some animals could have been affected more than others. Next, these dolphins were NOT the part of mass stranding and the majority stranded alone, especially the bottlenose ones. Finally authors also say that deaf dolphins can survive in social groups and thus being hearing impaired is not the reason for stranding.

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